Malware may generally refer to any form of malicious software, which can undesirably exist in a computer system. Categories of malware include, but are not limited to, viruses, worms, trojan horses, backdoors, and spyware. Viruses and worms may generally refer to software designed with a purpose of, among other things, replicating itself. Viruses typically utilize a host to spread whereas worms are typically stand-alone software. Trojan horses may generally refer to software that is disguised as legitimate software, but contains additional hidden code which, among other malicious activities, may allow the unauthorized collection, exploitation, falsification, or destruction of data. Backdoors may generally refer to software that allows access to computer systems bypassing normal authentication procedures. Spyware may generally refer to software that collects and sends information (such as browsing patterns in the more benign cases or credit card numbers in more malicious cases) about users, typically without explicit notification.
Virus scanning programs are utilized to detect and eliminate malware. To facilitate detection, virus scanning programs utilize malware definitions to scan a computer system. The malware definitions generally describe characteristics associated with a particular coding of malware. As new malware is developed, new malware definitions are determined.
In any particular computer system, there is a constant race between the arrival of the malware and the arrival of malware's corresponding definition. If the malware code arrives before the malware definitions, the malware may be capable of stealthing or hiding its existence from the virus scanning program—even after the definitions arrive.